Absorption spectroscopy uses the range of electromagnetic spectra in which a substance absorbs. In absorption spectroscopy light of a particular wavelength is passed through the sample. After calibration, the amount of absorption can be related to the sample concentration through the Beer-Lambert law. Examples of absorption spectroscopy include, for example, ultraviolet/visible (UV/VIS) absorption spectroscopy (most often performed on liquid samples to detect molecular content) and infrared (IR) spectroscopy (most often performed on liquid, semi-liquid (paste or grease), dried, or solid samples to determine molecular information, including structural information).
Spectroscopic analysis typically uses a spectrometer. The spectrometer typically includes a radiation source such as a deuterium, tungsten or xenon lamp capable of emitting radiation over a very broad range of wavelengths. The light is coupled into a sample held in a sample cell. The spectrometer filters the light emitted by the lamp, before or after coupling with the sample, by use of monochromators, filters, gratings, etc. A detector, for example, a photodiode, photomultiplier, photodiode array, or CCD array, quantifies the amount of light passed through (absorption) or emitted by (fluorescence) the sample to provide a detectable signal. While this equipment provides analysis flexibility, it also requires complicated and expensive light sources, gratings, monochromators and other components to take advantage of this flexibility.
Some experiments and tests do not need a spectrometer with full wavelength coverage, but may be performed using light in a single wavelength or only a few wavelengths. For example, applications in the oceanographic field include nutrient analysis, e.g. nitrite/nitrate requiring light at only 540 nm, phosphate requiring light at only 880 nm or 710 nm and iron requiring light at only 562 nm when using colorimetric techniques. Further, applications in biochemistry include protein detection, which could either be performed directly in the UV at 280 nm, or via colorimetric techniques, such as the modified Lowry Protein Assay, with detection at 650 nm (normalized at 405 nm) or the Bradford Assay, where the bound protein-dye complex is measured at 595 nm and can be normalized in the 700 to 750 nm region. Most of these analyses can be performed using single wavelength detection. Their accuracy could be improved by using a second wavelength for baseline offset and a third and/or fourth wavelength for simple absorbance shape detection to eliminate or indicate other colorimetric substances in the sample solution and correct for them. However, in some applications these improvements are not needed.
LEDs have long been used as quasi-monochromatic light sources. They are readily available for nearly all parts of visible light spectra. Recently UV LEDs with emission wavelength as low as 250 nm have become commercially available. Belz, M., Photonics West 2007. Many application specific LED-based detection systems have been developed and patented. Recently, an optical arrangement for assay strips was designed and disclosed in U.S. Pat. No. 7,315,378. It purportedly allowed the reliable reading of optical test strips and was based on several LEDs and photodetectors. These detection systems usually rely on single wavelength detection and may use a second photodiode to correct for the inherent drift behavior of the LED. A filterless chromatically variable light source, wherein light is coupled via optical fibers from several LEDs into a single optical fiber is disclosed in U.S. Pat. No. 5,636,303. The disclosed light source allowed individual control of the intensity of each LED to generate either light of a particular wavelength or a white light spectrum.